检查 GUID 是否为空(在 C 中)

Question

我想检查一个 GUID 结构是否为空/所有字段都为 0。这是我写的代码:

#include <windows.h>

static BOOL IsEmptyGuid(const GUID * const pGuid)
{
    return \
    (pGuid->Data1 == 0) &&
    (pGuid->Data2 == 0) &&
    (pGuid->Data3 == 0) &&
#ifdef _WIN64
    (*(DWORD64 *)pGuid->Data4 == 0);
#else
    (*(DWORD *)pGuid->Data4 == 0) && (*(DWORD *)(pGuid->Data4 + 4) == 0);
#endif
}

/* GUID definition from MSDN
typedef struct _GUID {
    DWORD Data1;
    WORD  Data2;
    WORD  Data3;
    BYTE  Data4[8];
} GUID;
*/


int main() {
    GUID guid1;
    guid1.Data1 = 0;
    guid1.Data2 = 0;
    guid1.Data3 = 0;
    memset(guid1.Data4, 0x0, 8);

    printf("Result: %u\n", IsEmptyGuid(&guid1));
}

检查字段 Data4 是否等于 0 的更安全方法是遍历每个字节并检查其值。但是，我发现上面的代码更具表现力。

我想知道，这是正确的吗？安全吗？

谢谢!

Answer 1

代码不正确。它违反了严格的别名规则(N1570 §6.5 p7)，导致未定义的行为。

An object shall have its stored value accessed only by an lvalue expression that has one ofthe following types: ⁸⁸⁾

• a type compatible with the effective type of the object,
• a qualified version of a type compatible with the effective type of the object,
• a type that is the signed or unsigned type corresponding to the effective type of theobject,
• a type that is the signed or unsigned type corresponding to a qualified version of theeffective type of the object,
• an aggregate or union type that includes one of the aforementioned types among itsmembers (including, recursively, a member of a subaggregate or contained union), or
• a character type.

_{88) The intent of this list is to specify those circumstances in which an object may or may not be aliased.}

确切地说，UB 发生在您使用非匹配类型取消引用指针时:

DWORD64 * temp = (DWORD64 *)pGuid->Data4; // Allowed, but implementation defined
DWORD64 temp2 = *temp;                    // Undefined behaviour

使用循环分别检查每个元素，或与 memcmp 比较相同大小的零填充数组。

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如评论中所述，某些编译器允许禁用严格别名，但应避免这样做，因为它会降低代码的可移植性，并且您仍然存在潜在的对齐问题。